The answer to that question may be found in a very detailed article at kernelthread.com.
Written by a recent Mac convert who was asked to give an overview of OS X to a local Linux users group, the article is very technical in nature and has exposed me to aspects of the system that I hadn’t previously known about. There’s enough material here to occupy many more hours exploring, learning and reconfiguring my system in search of that perfect Mac experience.
Table of Contents
History
* A Brief History of Mac OS X
Architecture
* Architecture of Mac OS X
* Booting Mac OS X
* XNU: The Kernel
* Above the Kernel
* Mac OS X System Startup
* Mac OS X Filesystems
Programming
* Programming on Mac OS X
Features
* A Sampling of Mac OS X Features
Software
* Available Software for Mac OS X
Conclusion
* Why Mac OS X?
Appendix
* Mac OS X Hacking Tools